This invention pertains to temperature measurement, and more particularly to measurement of temperatures across large spaces of known distance in a noisy, dirty and corrosive environment such as a coal-fired utility boiler, or a chemical recovery boiler.
Coal-fired boiler operations are significantly influenced by operational parameters that vary with changing environmental factors, including ambient temperature, humidity, coal composition, etc. Gas temperatures in the boiler, including furnace exit gas temperatures, are influenced by these factors, as well as by adjustments that can be made to the furnace apparatus, such as burner configuration and orientation, air flow rate, coal feed rate, etc.
Gas temperatures profoundly affect the performance of a boiler in several ways. The thermal NOx formation rate increases exponentially at temperatures over 2700xc2x0 F. There is strong regulatory pressure to reduce NOx emissions, but the fundamental knowledge of furnace exit gas temperatures, the primary factor in NOx formation, is lacking in large boilers because existing temperature measurement technology is incapable of producing accurate temperature data in large boilers.
Boiler gas temperatures also influence slag formation rates on boiler tubes. Slag is an accumulated deposit of materials present in coal that are formed as ash particles when the coal is burned in the furnace but which impinge and stick on the pendant steam/water tubes when the gas temperature is near the fusion temperature of the ash particles (the so-called xe2x80x9csticky zonexe2x80x9d). Slagging of the tubes is a common phenomenon in all coal fired boilers, but is particularly troublesome in those boilers using sub-bituminous coal such as the low sulfur coal from the huge deposits in the Power River Basin. Slag is a problem because it interferes with heat transfer to the boiler tubes, and can impede gas movement through the tube banks. Even more serious, slag deposits can grow to enormous size and then fall, causing extensive damage to the boiler and resulting in expensive boiler down-time while repairs are made.
Boiler tubes are cleaned of slag deposits by xe2x80x9csoot blowingxe2x80x9d blasts of steam injected through vents in rotating pipes, but the frequency and location of the soot blowing is based primarily on guesswork by the operator rather than a real knowledge of the actual current conditions in the boiler that produce slagging of the boiler tubes. Soot blowing reduces the efficiency of the boiler and can itself cause erosion of the tubes, so there is a strong incentive to optimize the soot blowing operation, that is, to operate it only with the necessary frequency, duration and location. One technique to determine when the tubes are becoming slagged is by measuring the temperature on opposite sides of a bank of tubes to ascertain how much heat is being transferred through the tubes to the water/steam in the tubes. When the temperature differential drops, that is an indication that the tubes are becoming slagged since the slag acts as an insulator, retarding the heat transfer. However, there must be an accurate measure of the gas temperatures on opposite sides of the tube banks for the temperature differential scheme to work, and state of the art temperature measurement techniques are inaccurate or short lived for large boiler installations.
A better approach to the slagging problem would be to minimize the formation of slag and thereby reduce the need to remove it. Since slag formation is influenced by gas temperature, a knowledge of the temperatures at the inlet plane to the tube bank, and/or in the tube bank itself would enable the boiler operator to determine when the thermal conditions are approaching those under which tubes are likely to become slagged. Control of gas temperature to prevent the creation of the xe2x80x9cstickyxe2x80x9d zone of slag formation would help to delay the onset of boiler pluggage and forced shutdown for cleaning. Detailed knowledge of the thermal conditions in the region of the tube bank can be helpful, not only in assessing where slagging is likely to occur, so that soot blowing may be optimized for the conditions, but also can be used in adjusting the furnace to produce gas temperatures which minimize slagging.
Thus, there has long been a need for accurate temperature measurements in large power and recovery boilers that enable improvements to be made in boiler efficiency, and also reduce the formation of slag and optimize soot blowing to remove slag that does form so that large slag deposits do not form and cause boiler damage from slag falls. The temperature measurement would also be useful in minimizing NOx formation by reducing the dwell time at high temperature. Finally, such a temperature measurement would facilitate furnace fireball centering, firewall flame impingement detection, and tube leak detection.
The long felt need for accurate temperature measurement in large boilers exists because the prior art measurement techniques are inadequate to reliably produce accurate temperature measurement across the width of large boilers. Thermocouple probes are unreliable and fail quickly in corrosive environments. Optical methods have limited penetration and are difficult to interpret. Prior acoustic methods cannot operate accurately over large spans in noisy environments, in part because they are unable to accurately detect the onset of the acoustic signal in high amplitude background noise.
Accordingly, it is an object of this invention to provide an improved acoustic pyrometer that can make accurate measurements of elevated gas temperatures across wide spaces in the presence of substantial acoustic noise. Another object of this invention is to provide an improved acoustic pyrometer capable of accurate operation in atmospheric conditions wherein the path length before absorption of optical wavelengths used in optical pyrometry are short and optical pyrometry is difficult to interpret. It is another object of this invention to provide an improved acoustic pyrometer having the capability of performing advanced diagnostic functions regarding the internal operation of a boiler, e.g. to facilitate optimal furnace adjustment for fireball centering, firewall flame impingement detection. It is yet another object of this invention to provide improved methods for measurement of heat transfer to boiler tubes, tube leak detection, localization of slag-formation regions, furnace plane temperature mapping, and optimizing soot blowing operations.
These and other objects of the invention are attained in an improved acoustic pyrometer for measuring the average gas temperature across an open space of a known distance. It includes an acoustic signal generator for generating an acoustic signal with a high amplitude sudden onset or short rise time. A detector is positioned adjacent the signal generator in a position to detect the onset of the acoustic signal in the signal generator. The detector could be an acoustic signal detector such as a microphone or a piezoelectric detector, or it could be a proximity or translation detector that senses the movement of the signal generator component that releases the acoustic signal. The detector generates a first electrical signal corresponding in time to the onset of the acoustic signal in the signal generator. A receiver is positioned across the space of known distance from the signal generator, and has a microphone or other acoustic signal sensor for receiving acoustic signals from the space and for generating electrical signals corresponding to amplitude and frequency of the acoustic signals received in the receiver. The signals from the acoustic signal detector associated with the signal generator and the acoustic signal sensor in the receiver are sent to a signal processor, to distinguish the acoustic signal from background noise in the space as detected in the receiver, and for comparing the time of arrival of the acoustic signal in the receiver with the time when the acoustic signal was generated in the signal generator to determine the transit time of the acoustic signal to traverse the space, and for calculating the temperature of the gas in the space based on the transit time.